Traditionally, in an injection molding machine, a metal mold (a fixed mold and a movable mold) is fixed on a platen such as a stationary platen and a movable platen and the movable platen is moved toward/away from the stationary platen to close/open the mold. With the mold closed, molten synthetic resin is injected in the cavity of the mold to form a molded article. Then, the mold is opened and the molded article is ejected by an ejector mechanism.
Here, the mold is fixed to the platen of an ejection molding machine using plural bolts or a hydraulic clamping device in many cases. Recently, magnetic fixing devices have been put into practical use, in which a clamping plate (magnet plate) in which plural magnetic force generation mechanisms are installed is attached to the platen and magnetic force is used to adsorb and fix the mold onto the fixing surface of the clamping plate.
In the injection molding machine of Patent Document 1, clamping plates in each of which plural magnet units (magnetic force generation mechanisms) are installed are fixed to the stationary platen and movable platen, respectively, and the magnetic force generated by the plural magnet units is used to fix the molds to the fixing surfaces of the clamping plates. The magnet units each have a steel block, plural permanent magnets arranged on the outer periphery of the steel block, an Alnico magnet placed on the back of the steel block, and a coil wound around the outer periphery of the Alnico magnet. The magnetic field direction of the Alnico magnet is reversed in accordance with the energizing direction of the coil.
For fixing a mold to the fixing surface of a clamping plate, the magnet units are powered from the control unit through operation using the operation panel. The coil is energized for several seconds to switch the magnetic field direction (polarity) of the Alnico magnet so that the Alnico magnet and permanent magnets together establish a magnetic circuit of which the magnetic path is partly formed by the mold, obtaining an adsorption state in which the magnet units adsorb the mold.
On the other hand, for releasing the mold, the operation panel is operated to energize the coil for several seconds in the opposite direction to that for adsorbing the mold to switch the polarity of the Alnico magnet so that a magnetic circuit of which the magnetic path does not include the mold in any part is established, whereby the non-adsorption state in which the magnet units do not adsorb the mold is obtained.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-169840.